1. Field of the Invention
The subject of the invention relates to an inductive power transfer system and more specifically to a synchronous system for inductively transferring power.
2. Description of the Prior Art
Inductive power transfer can be used to power a device and/or charge a remotely located battery without the need for any electrical connection between the device and a power source. Inductive power transfer typically uses a primary coil and a secondary coil. The primary coil may be contained within a primary unit or source connected to an AC (Alternating Current) power source. The secondary coil may be contained in a secondary or receiving system, which may be directly included in, for example a cordless consumer device. When the cordless device is placed near the primary unit, such that the primary coil is in proximity to the secondary coil, power is inductively transferred from the primary coil to the secondary coil. The energy stored by the secondary coil can be utilized to power and/or charge the consumer device.
One problem with inductive power transfer is the lack of spatial freedom between the primary coil and the secondary coil. The efficiency at which the primary coil transfers power to the secondary coil is limited by the distance between the primary unit and the device, including the secondary unit. Operating the primary coil and the secondary coil at mutual frequencies, at or close to resonance, increases the efficiency of the power transfer. It is additionally desirable to adjust the magnitude of oscillation existing at the primary coil to control the magnitude of power transfer from the primary coil to the secondary coil. However, adjusting frequency to operate at resonance and simultaneously controlling amplitude can be problematic. Additionally, using voltage pulses to control the resonance frequency and amplitude can easily generate undesired electromagnetic emissions when inductively coupling a primary coil to a secondary coil. Furthermore, primary and secondary coils intended to couple with a high degree of spatial freedom can easily radiate unwanted electromagnetic energy due to the pulsing form of the control, especially if the secondary coil includes a rectifier circuit having sudden changes in current flowing through the secondary coil. These electromagnetic energies may affect a variety of devices, such as radios in vehicles.